Field of the Invention
The present invention relates to pulsed plasma thruster systems. More particularly, the present invention relates to the field of engineering to develop a programmable and fault tolerant pulsed plasma thruster apparatus/system that can produce impulse-bits from a set of pulsed plasma thrusters, and that can be managed at the system component levels to provide redundancy features while allowing timing control of the impulse-bit production and monitoring functions at fractions of a single pulse time period.
Background of the Related Art
There is a broad area of pulsed propulsion system design, incorporating pulsed chemical thrusters, pulsed electric propulsion/micro-propulsion systems, pulsed plasma thruster, vacuum arc plasma thrusters, micro-cathode arc thrusters, and similar micropropulsion systems imparting small bursts of thrust, in the form of ‘impulse-bits’. The units of impulse-bits, is exactly similar to the unit of impulse, which is the product of a force F, and the time t, for which it acts. An example impulse-bit unit commonly seen in micropropulsion application is that of Micro-Newton Seconds, or μNs. Another definition of Impulse-Bit is that it is the smallest change in momentum required to allow for fine attitude and orbit control of a spacecraft.
Certain propulsion systems have control systems that use continuous series of rectangular trigger pulses (singular or plural arrangement) as control signals to actuate a system that produces impulse-bits, on the basis of rising/falling edges of an external trigger, and accuracy in the timing of impulse-bit generation is usually on the order of a full impulse-bit timing period, or allowed to drift due to the complex processes in generating plasma arc discharges.
Examples of arc thrusters are shown, for instance, in U.S. Patent No. Publ. 2011/0258981 to Keidar, and U.S. Pat. No. 6,818,853 to Schein. The thrusters are actuated, and produce impulse-bits, by the rapid discharge of medium current, low-voltage, micro-second scale energy pulses delivered at the anode-cathode terminals of a thruster device. The Plasma Power Unit (PPU) is utilized to store energy in regular timed intervals and to discharge the same energy in an extremely brief period, at a medium current level, based upon the time constants that can be calculated from fundamental electrical properties of the electrical energy/inductive-magnetic energy circuitry. In this invention, a driver trigger pulse is required to activate the PPU, and it is assumed that multiple channels in a set of thrusters (each, employing their own PPU, or a shared PPU) will be triggered with rectangular individual pulses. Minute adjustment of the triggers, within the time span of a single cycle, is not considered at the pulse-by-pulse level in both patents. The driver trigger pulses are generated in a simple manner, with the in nature and the timing/pulse-width duty cycle for any system chosen to match the physical response times of the PPU resistance, capacitance, inductor and power switching elements, typically after series of experimental trials, and this is due in part to the degree of difficulty observed in experimental trials to ensure that each external trigger event produces an equal and precise plasma impulse-bit output from the system. Possible impulse-bit variation due to minute variation in component values of the PPU circuit or electrical/thermal/mechanical behavior of the thruster head after repeated operations, and over lifetime of the components, is not given adequate consideration.
In any Vacuum Arc Thruster system, the efficiency and the thrust power levels that can be generated from each impulse-bit operation depends upon the pulse-width modulated energy from the PPU being discharged to the anode/cathode terminals of the thrusters. If the anode-cathode interfaces and thruster are completely enclosed in a standalone container, with only mechanical and passive elements, and a connection to a separate PPU, at any distance, they can then be referred to as ‘thruster-heads’, but in previous Keidar and Shein patent documentation, such a thruster-head has also been referred to as ‘thruster’. For the purposes of this document, Thruster-Head will refer to the stand-alone thruster device, whereas Thruster Channel will be used to refer to a combination of at least one PPU, at least one Thruster-Head, and any intermediary wiring. A ‘Thruster’ therefore will always be considered to be equivalent to a Thruster-Head, and be a vital part of a Thruster Channel.
The force represented by impulse-bit value that can be generated from a Thruster Head mechanism is a complex function of electrical and mechanical properties of the anode/cathode materials, their physical separation, the presence of a suitable impregnated catalyst, as insulator, to achieve micro-plasma, leading to generation of the actual plasma flow. The production process is also convoluted, due to the random “walking” motion of the rotating cathode spot and surface irregularities where the propulsion material being ablated. Once the plasma arc discharge process is underway, cathode terminal matter is converted from solid to plasma state and departs the discharge zone at high velocities at right angles to the electrical and magnetic fields. Detailed behavior of plasma thrusters are described in the Keidar and Schein patents.
Precision timing control of thruster operation of a spacecraft is vital for space missions, and also difficult to achieve due to the nature of the physical processes for generating thrust from a propellant and the available mechanisms to control that production (of thrust) effectively when only a small amount of impulse is desired. When a thruster fires longer (or more powerful), or shorter (or less powerful) than required by the calculations of a spacecraft dynamics computer, an error can be calculated and the next cycle of thruster operations has to be adjusted to minimize the cumulative error, towards acceptable limits, over a series of thruster firings. Onboard calculation of such error basis requires a suitable feedback mechanism indicating when a thruster has fired, in cue with a determination system that precisely estimates the position/attitude of the spacecraft, and perhaps a calibration guideline to indicate how far the measurements are off nominal values.
In U.S. Pat. No. 8,019,493 to Weigl et al., a spacecraft thruster torque ‘feedforward’ calibration system is discussed in which a plurality of thrusters (of an indeterminate nature) are triggered by firing commands and the resultant error is used to create a lookup table that would be used to create torque calibration. The reduction of total attitude error to micro-radian levels is performed by adjusting the thruster firing of future cycles by accommodating both measured and calibrated values of previous firings, using a dynamically updated table that has to be maintained between firings, per thruster set. Control methods are asserted in a manner that focuses on the triggering of the thruster, and measuring errors that occur afterwards, although fine adjustment of the trigger mechanism within a fractional time span of a single cycle, or programmable adjustment to account for further changes in system behavior would be a useful addition to this invention. The Weigl patent indicates the importance of measuring, analyzing and calculating the eventual time mark, and eventual force levels at which the thruster produced an impulse-bit, or impulse, and use that to as a prediction/correcting factor for future thruster firings.
Electric propulsion systems require different control mechanisms than Chemical propulsion systems. Chemical propulsion is used in the vast majority of launch vehicle rocket systems and missiles. There are occasional use-cases where a common need between the two domains can be identified, with regard to the production of several impulse-bits. Any thruster propulsion system has a certain inherent processing delay and processing characteristics that may change over time and exhibit different behavior in different modes of operation, and then there is a need to synchronize key events (e.g. a firing trigger, on-off states, fuel flow etc.). It would be beneficial to develop a method to fine tune the timing of the trigger signals so that the effective production time mark of the impulse-bit can be adjusted to account for potential system changes, after operating for repeated cycles and duration. In U.S. Patent Publ. No. 2010/0121552 to Le Gonidec et al., an engine is adjusted by the use of slow valves, to bring the rocket engine to the operating point that complies with the set points, and so as to keep it there. In the Le Gonidec publication, a fine adjustment method, precise to within a small fraction of a single impulse-bit pulse cycle is described.
A vacuum arc thruster as described in U.S. Pat. No. 7,518,085 to Krishnan, is actuated by using a combination capacitive driver and inductive-energy storage system to generate a voltage breakdown across a very small gap at a fairly constant voltage, in pulses at the millisecond and microsecond scale, which are triggered with digital logic. The combination of a single PPU is considered for both single-thruster head per channel, or multi-thruster-head per channel use. Different geometry possibilities are explained, and throttle control is proposed by changing the repetition rate of the trigger pulse, or changing the duty cycle of current applied to the energy storage element is covered. Synchronization between individual PPU of a set of PPU of this invention is not discussed by Krishnan. In the accompanying U.S. Pat. No. 7,053,333 to Schein et al., associated with the aforesaid Krishnan vacuum arc thruster patent, a switched trigger pulse (referred as SW_ON) is utilized/Similarly, in an earlier pulsed thruster system, described in U.S. Pat. No. 6,735,935 to Hruby et al., the operation of a control unit which drives a processing unit or propellant storage and delivery system, or both of them, is discussed with two types of constraints: repetitive pulse widths having constant duration, frequency, or constant duration and variable frequency. In neither of the above mentioned patents and patent applications has the possibility of a fractional unit of impulse-bit, say 1.5 impulse-bits production requirement be considered, rather than in whole impulse-bit units like 1, 2, 3, . . . any number of integer units.
A spacecraft is usually power limited, and also constrained in power storage capability. Arc thrusters are mostly power hungry devices. With respect to the all-important energy needs of such an electric propulsion system, there is a well-recognized need to manage energy budget effectively to maintain electric propulsion thruster operation in a pulsed/continuous mode. For example, in U.S. Pat. No. 6,581,880 to Randolph et al., burn times, as calculated using orbit analysis, are used as a control variable and compared to a power analysis that results in determination of allowed thruster voltage, current and power draw, and a maximum period in which the EP device can be operated before it is shutdown. This method works when a single EP device is used, however, in a bank of EP devices an additional synchronization fabric and logic switching will surely need to be implemented in order to smooth the maximum load on the spacecraft circuits, and that possibility does not seem to be presented in the patent. The lack of consideration of the possible benefits or requirements for fine adjustment of the operation of a set of pulsed thrusters is also seen in other earlier electric propulsion inventions, an example of which is U.S. Pat. No. 5,947,421 to Beattie et al., where it is succinctly described that the invention is particularly suited for spacecraft in which only one thruster is fired at any given time.
If the scope of investigation is now shifted to reviewing three axis thruster modulation inventions, such as can be seen in U.S. Pat. No. 5,310,143 to Yocum et al., a thruster select and timing logic requirement is described in general terms, where it is succinctly admitted that the issue is, “a difficult task”, and that there many ways to accomplish such functions. It is also stated that a precise method is not part of the invention, and that any such type of control (e.g., select and timing logic) would have to have the ability to use spacecraft mass properties with thruster placement and alignment information to select thrusters and compute the necessary on-time commands. It is stated that only then, can the on-times be adjusted to fit the system timing characteristics, allowing for the possibility of multiple thrusters in use simultaneously, but no explicit argument is made for implementing fine synchronization of thruster operations at fractions of time smaller than the invention's sample period, which is necessary to ensure precision operation of a multi-channel thruster application. Adjusting time marks in fractions of a impulse-bit time-period is a necessary part of a process to ensure higher degrees of precision in spacecraft operations.
On the other hand, one example of a method for fine adjustment of multiple trigger signals that need to be ‘phased’ in relation to key events can be seen in U.S. Pat. No. 5,369,564 to Choi, and the invention example concerns a phase-difference synchronization controlling circuit of a power circuit for operating in parallel two or more switched mode power supplies. While the example invention does not claim any application to the operation of a vacuum arc thruster, it lends itself to the understanding by any skilled in the electrical domain that a spacecraft PPU for a pulsed plasma thruster could be logically treated as a complex power conversion system, almost like a switched mode power supply where power input is ‘processed’ and ‘converted’ to a power output, in functionality, and therefore could be treated as a device where the operation of its various parts can be minutely adjusted to provide any preferred ‘mode’.
In the general area of time-based triggering, an example of such an invention in which the delayed signal closest in time to a reference signal (or, key event), is selected as the output signal, is seen in U.S. Pat. No. 4,600,895 to Landsman. But for a spacecraft utilizing vacuum arc thrusters that may need minute adjustment in their operation across a time-window, throughout their long duration lifetime expectancy, it can be estimated that there may be future situations where it will be necessary to adjust the triggering of the device(s) either before (a special case) or after a key events (more common case), and this dual requirement is not covered by Landsman. In addition, in the mentioned invention, no discussion is made of the possibility that there might have to be an independent high resolution timing reference source to establish a accurate time base for the fine adjustment of trigger pulse controlling plasma phenomena, that typically last only micro-seconds and smaller time intervals.
It can therefore be stated that existing synchronization methods from other domains remain to be extended to the very precise domain of plasma engineering based vacuum arc thrusters and their derivative inventions, to allow fine precision operation. Such a utility invention would have the potential to benefit other inventions such as those described in U.S. Pat. No. 4,161,780 to Rudolph et al., where a requirement to precisely control the delayed firing of thruster jets to correct or adjust the attitude of a spinning spacecraft is detailed.